Mock-like expectations for TestChainSource

Add a method to TestChainSource to test chain::Filter expectations. This
is limited to register_output, allowing tests to assert that the method
was called with a specific output and dictate what the return value is.

Multiple expectations are checked in the order in which they were added.
Failure occurs if a call doesn't match the next expectation or if there
are unsatisfied expectations. If not expectations are added, then no
calls are checked.

lightning/src/util/test_utils.rs

@@ -39,7 +39,7 @@ use std::time::Duration;
 use std::sync::{Mutex, Arc};
 use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
 use std::{cmp, mem};
-use std::collections::{HashMap, HashSet};
+use std::collections::{HashMap, HashSet, VecDeque};
 use chain::keysinterface::InMemorySigner;
 
 pub struct TestVecWriter(pub Vec<u8>);
@@ -518,6 +518,7 @@ pub struct TestChainSource {
 	pub utxo_ret: Mutex<Result<TxOut, chain::AccessError>>,
 	pub watched_txn: Mutex<HashSet<(Txid, Script)>>,
 	pub watched_outputs: Mutex<HashSet<(OutPoint, Script)>>,
+	expectations: Mutex<Option<VecDeque<OnRegisterOutput>>>,
 }
 
 impl TestChainSource {
@@ -528,8 +529,17 @@ impl TestChainSource {
 			utxo_ret: Mutex::new(Ok(TxOut { value: u64::max_value(), script_pubkey })),
 			watched_txn: Mutex::new(HashSet::new()),
 			watched_outputs: Mutex::new(HashSet::new()),
+			expectations: Mutex::new(None),
 		}
 	}
+
+	/// Sets an expectation that [`chain::Filter::register_output`] is called.
+	pub fn expect(&self, expectation: OnRegisterOutput) -> &Self {
+		self.expectations.lock().unwrap()
+			.get_or_insert_with(|| VecDeque::new())
+			.push_back(expectation);
+		self
+	}
 }
 
 impl chain::Access for TestChainSource {
@@ -548,7 +558,71 @@ impl chain::Filter for TestChainSource {
 	}
 
 	fn register_output(&self, output: WatchedOutput) -> Option<(usize, Transaction)> {
+		let dependent_tx = match &mut *self.expectations.lock().unwrap() {
+			None => None,
+			Some(expectations) => match expectations.pop_front() {
+				None => {
+					panic!("Unexpected register_output: {:?}",
+						(output.outpoint, output.script_pubkey));
+				},
+				Some(expectation) => {
+					assert_eq!(output.outpoint, expectation.outpoint());
+					assert_eq!(&output.script_pubkey, expectation.script_pubkey());
+					expectation.returns
+				},
+			},
+		};
+
 		self.watched_outputs.lock().unwrap().insert((output.outpoint, output.script_pubkey));
-		None
+		dependent_tx
+	}
+}
+
+impl Drop for TestChainSource {
+	fn drop(&mut self) {
+		if std::thread::panicking() {
+			return;
+		}
+
+		if let Some(expectations) = &*self.expectations.lock().unwrap() {
+			if !expectations.is_empty() {
+				panic!("Unsatisfied expectations: {:?}", expectations);
+			}
+		}
+	}
+}
+
+/// An expectation that [`chain::Filter::register_output`] was called with a transaction output and
+/// returns an optional dependent transaction that spends the output in the same block.
+pub struct OnRegisterOutput {
+	/// The transaction output to register.
+	pub with: TxOutReference,
+
+	/// A dependent transaction spending the output along with its position in the block.
+	pub returns: Option<(usize, Transaction)>,
+}
+
+/// A transaction output as identified by an index into a transaction's output list.
+pub struct TxOutReference(pub Transaction, pub usize);
+
+impl OnRegisterOutput {
+	fn outpoint(&self) -> OutPoint {
+		let txid = self.with.0.txid();
+		let index = self.with.1 as u16;
+		OutPoint { txid, index }
+	}
+
+	fn script_pubkey(&self) -> &Script {
+		let index = self.with.1;
+		&self.with.0.output[index].script_pubkey
+	}
+}
+
+impl std::fmt::Debug for OnRegisterOutput {
+	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+		f.debug_struct("OnRegisterOutput")
+			.field("outpoint", &self.outpoint())
+			.field("script_pubkey", self.script_pubkey())
+			.finish()
 	}
 }